1. Field of the Invention
The present invention relates to an organic light emitting device, and more particularly, to an organic light emitting device that includes an emission layer made of a phosphorescent dopant having a long lifetime and having excellent luminous efficiency.
2. Description of the Related Art
Emission materials used to form organic light emitting devices are divided into fluorescent materials using singlet excitons and phosphorescent materials using triplet excitons. Phosphorescent materials are organic metal compounds having heavy atoms. In phosphorescent materials, the relaxation of triplet excitons, which is a forbidden transition, occurs thus emitting light. In fluorescent materials, the singlet excitons having a generation probability of 25% are used. Accordingly, phosphorescent materials using triplet excitons having a generation probability of 75% have a better luminous efficiency than fluorescent materials.
An emission layer made of a phosphorescent material includes a host and a dopant. The dopant receives energy from the host, thus emitting light. Such dopants have been continually developed. For example, Princeton University and South California University have developed materials using an Ir metal compound.
Recently, an emission layer made of a carbazol compound as a host has been developed. The carbazol compound has a triplet-state energy band gap as compared to 4,4′-biscarbazolylbiphenyl (CBP). However, when known carbozol compounds are used to form phosphorescent devices, the efficiency and lifetime of the devices are far below a desirable level. Therefore, what is needed is an improved phosphorescent emission layer that has good luminous efficiency and long lifetime.